The diagnostic accuracy of invasion to the thoracic aorta (n = 23) and the superior vena cava (n = 22) on CT and MRI was compared to operative or autopsied finding in lung cancer. MRI was slightly superior to CT in its ability to diagnose tumor invasion to them. It is noteworthy that this invasion could be easily demonstrated by multiplanar MRI. In conclusion, because CT or MRI has diagnostic weak point respectively, these modalities should be combined adequately corresponding to the areas of tumor invasion.